The sinusoidal control of AC machines is commonly achieved by using the well-known vector control (also called field-orientated control) theory. It has been widely used for variable speed control of 3-phase asynchronous induction motors and synchronous PM AC machines. It delivers sinusoidal control of 3-phase current of AC motors with low torque ripple. Due to the involvement of extensive computation, a high-speed micro-controller, DSP or microprocessor (MCU) is usually required. Accurate rotor position information is essential for achieving high-performance vector control. However, high-resolution position sensors are usually fragile, unreliable and expensive. Therefore the development of position-sensorless sinusoidal control is driven by eliminating the cost and reliability problem caused by high-resolution position sensors. Position sensorless AC control is preferred for many industrial applications. Unfortunately accurate position determination is extremely difficult at start-up and very low speed due to the lack of position-dependent signals. Also, a high-performance MCU is required for sensorless methods due to the implementation of complex algorithms as well.
Low-cost switching-type Hall sensors are commonly employed to generate the commutation signals for controlling brushless DC motors in conventional six-step square-wave drives. They have also been proposed for a low-cost sine-wave drive. However, due to their rough position measurement, smooth torque control for PMSM at very low speed is not achievable.
Alternatively linear Hall sensors can provide more accurate position measurement continuously. Linear Hall sensors are widely utilised for measuring magnetic field strength. U.S. Pat. No. 6,522,130 proposes mounting a ring magnet on a rotor with two linear Hall sensor fixed on the stator with a phase displacement with 90 electrical degrees. The sine/cosine signal outputs from the two Hall sensors can be directly used to decode rotor position by using the well-known resolver-to-digital (RD) conversion as the process for resolver. Therefore, accurate and low-cost position determination is achievable if the magnetic flux distribution of ring magnet is sinusoidal. As a result, a sine-wave drive based on vector control theory can be implemented with a relatively low-cost solution.
All the sine-wave control strategies mentioned above can achieve the rapid and smooth torque control of AC machines by using a high-performance MCU. They are suitable for the high-performance servo systems. However, for some industrial applications, strictly rapid torque response is not necessary and only the torque smoothness of PMSM motor is the main concern. Furthermore the cost for high-performance MCU is intolerable in these applications. Therefore a sine-wave drive with even less cost is preferred.
A low-cost open-loop sine-wave solution for AC induction asynchronous motors can be implemented by well-known V/F (Voltage/Frequency) control without requiring position sensors and current sensor as well. Only low-end MCU or no MCU is required in these low-cost low-performance AC drives. The rotor magnetic field of the induction motors is induced due to the slip of rotor frequency and stator current synchronous frequency. The torque is generated by the interaction of the induced field in rotor and stator control flux. However, PMSM machines have no such feature. The rotor position of a PMSM has to be referred to for proper control of 3-phase stator currents in order to synchronise with the rotating magnetic field on the rotor of PMSM. Therefore simply applying the conventional V/F signal into the stator winding of PMSM is unable to guarantee the proper operation of a PMSM.
Emura T., Wang L., Yamanaka M., and Nakamura H., “A High-Precision Positioning Servo Controller Based on Phase/Frequency Detecting Technique of Two-Phase-Type PLL”, IEEE Transactions on Industrial Applications, VOL. 47, NO. 6, pp 1298-1306, December 2000 propose the use of a Two-Phase-Type PLL in the high precision servo control of motor used in a gear grinding machine.
The present invention seeks to provide a controller for a PMSM which can be implemented at low cost and operate as an open loop controller (or in a closed loop control system).